Process variable transmitters are used to measure process parameters in a process control or monitoring system. Microprocessor-based transmitters often include a sensor, an analog-to-digital converter for converting an output from the sensor into a digital form, a microprocessor for compensating the digitized output, and an output circuit for transmitting the compensated output. Currently, this transmission is normally done over a process control loop, such as a 4-20 mA control loop, or wirelessly.
One exemplary parameter that is measured by such a system is pressure, which is sensed by measuring the capacitance of a capacitance-based differential pressure sensor. Of course, these types of pressure sensors are only exemplary and others can be used as well. Similarly, pressure is only one exemplary process variable and a wide variety of other process control parameters can be measured as well, such as flow, pH, temperature, level, etc. Therefore, while the present discussion proceeds with respect to a pressure sensor, it will be appreciated that this discussion could just as easily proceed with respect to other sensors.
Process variable transmitters are often subjected to various types of noise, which can affect the accuracy of the measurement circuit. A significant source of noise in the analog-to-digital (A/D) converter is synchronous noise. Synchronous noise is noise that occurs at the same relative time in the A/D conversion process, on repetitive conversions. An example of this type of disturbance is bus noise produced by a microprocessor. If the A/D converter is synchronous with the processor bus, this type of noise can produce an offset error in the A/D converter output.
Various kinds of process variable transmitters are subjected to electromagnetic compatibility (EMC) testing procedures which look for susceptibility to disturbances produced in conducted radio frequency (RF) immunity tests. Synchronous noise is one mechanism responsible for a process variable transmitter being susceptible to this type of disturbance.
In this type of EMC testing procedure, common mode voltage noise is applied to the loop wiring relative to the module housing of the transmitter. This voltage shows up in the measurement circuit as a result of capacitive strays between the housing and the sensor. When the A/D converter samples the sensor signal, it samples noise coupling through the strays as well, such that noise at or near harmonics of the sampling frequency show up in the base band, and show up as measurement error. The system response of a process variable transmitter to this type of noisy input occurs in two different categories. The first is a wide band error and the second is a narrow band error. Narrow band errors are produced when interference feeds into the sensor sampling circuit at or near the sensor sampling frequency or harmonic. This results in aliasing errors in the base band at very low frequencies (close to direct current or DC).